Oil shale is a natural sedimentary rock containing an abundance of residual organic material which, when processed, can be made into oil and fuel products. Typically, oil shale, such as exemplified by the Green River formation in Wyoming, Colorado and Utah, has about 15-20% organic material embedded in an inorganic mineral matrix. The organic portion is composed generally of a soluble bitumen fraction and an insoluble fraction in which kerogen constitutes the bulk of the insoluble organic material. The bitumen fraction is readily solubilized by organic solvents and can be removed for refinement by physical means. The kerogen portion is characterized by its insolubility in organic solvents and is therefore more difficult to remove. In Green River oil shale, kerogen makes up about 75% of the organic components and in most all oil shale is the major organic component.
The inorganic mineral matrix in which the desired organics are trapped is composed primarily of carbonate materials such as dolomite and calcite, quartz and silicate minerals such as analcite or other zeolites, and will also usually contain substantial amounts of pyrite.
Several approaches have been used with oil shale for separating the organics from the mineral matrix. The usual process comprises crushing the matrix rock and subjecting the crushed matrix to heat in a retort to distill off the kerogen. Other processes involve erosion of the inorganics, for example by acid leaching, to keep the organics intact. Regardless of the method utilized, the kerogen retains a substantial amount of pyrite (iron sulfide) impurities. Such impurities form a major source of air pollution by sulfur dioxide during combustion. Many strong acids (e.g., hydrochloric, hydrofluoric or sulfuric acids) cannot dissolve pyrite from oil shales. While concentrated nitric acid can dissolve pyrite, it causes oxidation and nitration of the kerogen matrix. Pyrite has been removed by treatment of kerogen concentrate with lithium aluminum hydride in tetrahydrofuran solution at reflux temperature but with specific alteration of kerogen functional groups.
The present invention provides a process for removing pyrite from bituminuous material, preferably kerogen-containing material, which does not adversely affect the organic residue. Specifically, an electrolytically active slurry of the material is formed and placed in the anode chamber of a cell having a cathode chamber electrolytically operative therewith. Substantially non-oxidative electrolysis is conducted by using a neutral salt electrolyte and/or by operation at low electrolyte concentrations, less than 1.0 N. The pyrite is electrolytically reacted, resulting in substantial removal of pyrite from the material. The electrolysis is preferably conducted at a current density above about 50 amperes per square meter of anode surface (50 A/m.sup.2) for a period of at least half an hour until the pH of the slurry is reduced to less than about 1.5.
The process will be described with respect to the electrolysis of oil shale and of kerogen concentrate obtained therefrom, but is also applicable to coal, tar sands and other carbonaceous bitumens.